1. Field of the Invention
The present invention relates to a hopper structure for containing and treating loose materials, and particularly granular materials, which is equipped with an insert designed to affect the behavior of granular material contained therein, especially during an unloading step.
2. Background Art
The term “hopper” in the present description and in the claims refers to any type of container, both opencast or closed at the top (in which case it is sometimes termed silo), being variously shaped in cross-section, e.g. having a circular, squared or rectangular cross-section, and ending at the bottom thereof with a tapered discharging portion provided with a suitable delivery mouth, usually controlled by a suitable exhaust valve. It is well known that a hopper is generally loaded with loose material at its top portion and the material charged therein is delivered through its delivery mouth at the bottom of its tapered portion.
During an unloading step of the material loaded in a hopper, the downwards flow of loose material can be, in general, of two types: “mass” or “funnel” flow.
When “mass flow” is established, there occurs a uniform descent of all the material inside the hopper with no formation of preferential paths. Otherwise stated, the moduli of speed vectors of the various granules in the hopper at a right cross-section plane of the hopper are, if not identical, very similar to one another.
When “funnel flow” is established, there is, instead, non-uniformity in the values of speed vector moduli of the various granules along a hopper's right cross-section. More particularly, speed vector moduli at the central portion of the hopper at the same cross-section have a clearly greater value than the speed vector moduli of granules close to the hopper sidewalls. This phenomenon is indicative of the fact that at least one descending preferential path has been established in the material at least at the central portion of the hopper.
In many applications where a hopper is used as a treatment chamber for a granular material, e.g. in the processing of plastic materials reduced into granules (where “granules” is intended also to include flakes, scales and the like), it is essential to be able to guarantee a “mass flow” descent of the material loaded therein.
It is well known that, in processing many plastic materials, a very important treatment is the dehumidification of plastic material granulates, i.e. the removal of the water from within the granules of the polymeric materials termed “hygroscopic”. The removal of humidity from hygroscopic granules is necessary because, during melting of granular material being processed at relatively high temperature, any water possibly remaining within granules can slip into the polymer molecular chains, thus breaking them. Chain breaking results in a final product having much less than optimum mechanical characteristics because even blisters, blowholes, non-homogeneous coloring and other undesired phenomena often occur.
Granular plastic materials to be dehumidified are typically stored in hoppers or silos that are set in fluid communication with a hot and dried air generating device, generally termed “dryer”, that is designed to blow hot and dried air (processing air) into the hopper. Once inside the hopper, the processing air flows through the whole mass of granules of plastic material to be dehumidified, or part of it, removes humidity therefrom and comes out the hopper through a suitable outlet duct.
Reaching the desired dehumidification degree in a given granular plastic material, that will subsequently undergo melting in a processing machine (press) depends upon many factors. One of the most important factors is undoubtedly the dwelling time of the granular material in the dehumidification hopper. Depending on the dehumidification degree required for a given granular material to be treated, granules of plastic material have to be stored in the hopper for a determined and specific time interval (dwelling time).
In order to obtain homogeneous drying of a given plastic granular material loaded in a hopper, assuming the air to be distributed in a homogeneous way inside the hopper, the granular material must dwell inside the hopper for a dwelling time which is, in general, specific and typical for each plastic material.
The objective of ensuring the same dwelling time inside the hopper for all granules of a material, implies that, while the granular material descends in the dehumidification hopper, vertical components of the granule speed field are constant on the whole right cross-section of the hopper. As stated above, this type of flow is what is referred to as “mass flow” in the technical literature.
On the contrary, those flow configurations which are responsible for slowing down, or forming accumulations of granular material close to the hopper sidewalls (funnel flow), and causing accelerations in other more central areas of the hopper are to be avoided. A flow configuration of this type leads to the formation of plastic granules which have humidity values that differ from one another, and once fed into a processing machine (press) leads to products of poor quality.
In order to ensure a descending flow as much as possible of the “mass flow” type, it has already been suggested to provide a central conical insert member (with conicity facing upwards) in the hopper at the upper part of a lower tapered portion of the hopper or an elongated tubular insert, like that disclosed and illustrated in U.S. Pat. No. 6,405,454 (Kramer et al.). It is a foraminated insert closed at the bottom thereof and fed with hot and dried air at its top portion, and thus the air does not reach the whole mass of granular material loaded in the hopper. Moreover, under such an insert and above the hopper delivery mouth a “dead” zone is formed which is not reached by dried treatment air where granules, owing to a consequent drop in temperature, may absorb humidity, thereby affecting the physical-chemical characteristics of the product obtained once the granules are processed (moulded). In addition, an undesired “funnel flow” is generated in the dead zone during the discharge step, which results in the granules being mixed up again.